A microdevice for multiplexed detection of T-cell-secreted cytokines

Miniature enzyme-based electrodes for detection of hydrogen peroxide release from alcohol-injured hepatocytes

Alcohol insult to the liver sets off a complex sequence of inflammatory and fibrogenic responses. There is increasing evidence that hepatocytes play a key role in triggering these responses by producing inflammatory signals such as cytokines and reactive oxygen species (ROS). In the present study, we employed a cell culture/biosensor platform consisting of electrode arrays integrated with microfluidics to monitor extracellular H(2)O(2), one of the major ROS types, produced by primary rat hepatocytes during alcohol injury. The biosensor consisted of hydrogel microstructures with entrapped horseradish peroxidase (HRP) immobilized on an array of miniature gold electrodes. These arrays of sensing electrodes were integrated into microfluidic devices and modified with collagen (I) to promote hepatocyte adhesion. Once seeded into the microfluidic devices, hepatocytes were exposed to 100 mM ethanol and the signal at the working electrode was monitored by cyclic voltammetry (CV) over the course of 4 h. The CV experiments revealed that hepatocytes secreted up to 1.16 μM H(2)O(2) after 3 h of stimulation. Importantly, when hepatocytes were incubated with antioxidants or alcohol dehydrogenase inhibitor prior to alcohol exposure, the H(2)O(2) signal was decreased by ~5-fold. These experiments further confirmed that the biosensor was indeed monitoring oxidative stress generated by the hepatocytes and also pointed to one future use of this technology for screening hepatoprotective effects of antioxidants.

Print-to-print: a facile multi-object micro-patterning technique

In recent years, micropatterning techniques have gained increasing popularity from a broad range of engineering and biology communities for the promise to establish highly quantitative investigations on miniature biological objects (e.g., cells and bacteria) with spatially defined microenvironments. However, majority of the existing techniques rely on cleanroom-based microfabrication and cannot be easily extended to a regular biological laboratory. In this paper, we present a simple versatile printing-based method, referred to as Print-to-Print (P2P), to form multi-object micropatterns for potential biological applications, along with our recent efforts to deliver out-of-cleanroom microfabrication solutions to the general public (Zhao et al. 2009), (Xing et al. 2011), (Wang et al. 2009), (Pan and Wang 2011), (Zhao et al. 2011). The P2P method employs only a commercially available solid-phase printer and custom-made superhydrophobic films. The entire patterning process does not involve any thermal or chemical treatment. Moreover, the non-contact nature of droplet transferring and printing steps can be highly advantageous for sensitive biological uses. Using the P2P process, a minimal feature resolution of 229 ± 17 μm has been successfully demonstrated. In addition, this approach has been applied to form biological micropatterning on various substrates as well as multi-object co-patterns on the commonly used surfaces. Finally, the reusability of superhydrophobic substrates has also been illustrated.

An integrated microfluidic platform for in situ cellular cytokine secretion immunophenotyping

Rapid, quantitative detection of cell-secreted biomarker proteins with a low sample volume holds great promise to advance cellular immunophenotyping techniques for personalized diagnosis and treatment of infectious diseases. Here we achieved such an assay with the THP-1 human acute moncytic leukemia cell line (a model for human monocyte) using a highly integrated microfluidic platform incorporating a no-wash bead-based chemiluminescence immunodetection scheme. Our microfluidic device allowed us to stimulate cells with lipopolysaccharide (LPS), which is an endotoxin causing septic shock due to severely pronounced immune response of the human body, under a well-controlled on-chip environment. Tumor necrosis factor-alpha (TNF-α) secreted from stimulated THP-1 cells was subsequently measured within the device with no flushing process required. Our study achieved high-sensitivity cellular immunophenotyping with 20-fold fewer cells than current cell-stimulation assay. The total assay time was also 7 times shorter than that of a conventional enzyme-linked immunosorbent assay (ELISA). Our strategy of monitoring immune cell functions in situ using a microfluidic platform could impact future medical treatments of acute infectious diseases and immune disorders by enabling a rapid, sample-efficient cellular immunophenotyping analysis.

Aptamer-containing surfaces for selective capture of CD4 expressing cells

Aptamers have recently emerged as an excellent alternative to antibodies because of their inherent stability and ease of modification. In this paper, we describe the development of an aptamer-based surface for capture of cells expressing CD4 antigen. The glass or silicon surfaces were modified with amine-terminated silanes and then modified with thiolated RNA aptamer against CD4. Modification of the surface was first characterized by ellipsometry to demonstrate assembly of biointerface components and to show specific capture of recombinant CD4 protein. Subsequently, surfaces were challenged with model lymphocytes (cell lines) that were either positive or negative for CD4 antigen. Our experiments show that aptamer-functionalized surfaces have similar capture efficiency to substrates containing anti-CD4 antibody. To mimick capture of specific T-cells from a complex cell mixture, aptamer-modified surfaces were exposed to binary mixtures containing Molt-3 cells (CD4+) spiked into Daudi B cells (CD4-). 94% purity of CD4 cells was observed on aptamer-containing surfaces from an initial fraction of 15% of CD4. Given the importance of CD4 cell enumeration in HIV/AIDS diagnosis and monitoring, aptamer-based devices may offer an opportunity for novel cell detection strategies and may yield more robust and less expensive blood analysis devices in the future.

Simultaneous detection of cell-secreted TNF-α and IFN-γ using micropatterned aptamer-modified electrodes

Cellular production of such cytokines as interferon (IFN)-γ and tumor necrosis factor (TNF)-α is used to determine disease-specific immune responses and may be used to diagnose infectious diseases such as tuberculosis. In this paper, we describe the development of micropatterned electrodes functionalized with electroactive aptamers for multiplexed detection of immune-cell-produced cytokines. A sequence of electrode deprotection and aptamer incubation steps were used to assemble anti-IFN-γ DNA aptamers and anti-TNF-α RNA aptamers on individually addressable half-ring electrodes. Aptamer molecules were thiolated for assembly on gold and were functionalized with methylene blue redox reporter for electrochemical signal transduction. Specificity of individual sensors to the correct cytokine species was confirmed by exposure to recombinant cytokines. For cell detection experiments, electrode arrays were integrated into microfluidic devices and incubated with immune cells. Design of the surface was such that a small group of ~400 cells attached in the circular adhesion sites surrounded by half-ring electrodes sensing IFN-γ and TNF-α. The microdevice consisted of two parallel microfluidic channels, each channel containing four cell capture/sensing sites. Upon mitogenic activation, secreted IFN-γ and TNF-α molecules were monitored by performing square wave voltammetry (SWV) at different time points at individually addressable electrodes. This biosensing platform was used to analyze the quantity and rate of cytokine release from primary T cells and a monocyte cell line. Upon further development of this platform may be enhanced to enable detection of larger number of cytokines and used to correlate the levels and dynamics of cytokine release in immune cells to diagnosis and treatment of infectious diseases.

Affinity and enzyme-based biosensors: recent advances and emerging applications in cell analysis and point-of-care testing

The applications of biosensors range from environmental testing and biowarfare agent detection to clinical testing and cell analysis. In recent years, biosensors have become increasingly prevalent in clinical testing and point-of-care testing. This is driven in part by the desire to decrease the cost of health care, to shift some of the analytical tests from centralized facilities to "frontline" physicians and nurses, and to obtain more precise information more quickly about the health status of a patient. This article gives an overview of recent advances in the field of biosensors, focusing on biosensors based on enzymes, aptamers, antibodies, and phages. In addition, this article attempts to describe efforts to apply these biosensors to clinical testing and cell analysis.

Biosensors for immune cell analysis-A perspective

Massively parallel analysis of single immune cells or small immune cell colonies for disease detection, drug screening, and antibody production represents a "killer app" for the rapidly maturing microfabrication and microfluidic technologies. In our view, microfabricated solid-phase and flow cytometry platforms of the future will be complete with biosensors and electrical/mechanical/optical actuators and will enable multi-parametric analysis of cell function, real-time detection of secreted signals, and facile retrieval of cells deemed interesting.

A quartz nanopillar hemocytometer for high-yield separation and counting of CD4(+) T lymphocytes

We report the development of a novel quartz nanopillar (QNP) array cell separation system capable of selectively capturing and isolating a single cell population including primary CD4(+) T lymphocytes from the whole pool of splenocytes. Integrated with a photolithographically patterned hemocytometer structure, the streptavidin (STR)-functionalized-QNP (STR-QNP) arrays allow for direct quantitation of captured cells using high content imaging. This technology exhibits an excellent separation yield (efficiency) of ~95.3 ± 1.1% for the CD4(+) T lymphocytes from the mouse splenocyte suspensions and good linear response for quantitating captured CD4(+) T-lymphoblasts, which is comparable to flow cytometry and outperforms any non-nanostructured surface capture techniques, i.e. cell panning. This nanopillar hemocytometer represents a simple, yet efficient cell capture and counting technology and may find immediate applications for diagnosis and immune monitoring in the point-of-care setting.

Rapid, multiparameter profiling of cellular secretion using silicon photonic microring resonator arrays

We have developed a silicon photonic biosensing chip capable of multiplexed protein measurements in a biomolecularly complex cell culture matrix. Using this multiplexed platform combined with fast one-step sandwich immunoassays, we perform a variety of T cell cytokine secretion studies with excellent time-to-result. Using 32-element arrays of silicon photonic microring resonators, the cytokines interleukin-2 (IL-2), interleukin-4 (IL-4), interleukin-5 (IL-5), and tumor necrosis factor alpha (TNFα) were simultaneously quantified with high accuracy in serum-containing cell media. Utilizing this cytokine panel, secretion profiles were obtained for primary human Th0, Th1, and Th2 subsets differentiated from naïve CD4+ T cells, and we show the ability to discriminate between lineage commitments at early stages of culture differentiation. We also utilize this approach to probe the temporal secretion patterns of each T cell type using real-time binding analyses for direct cytokine quantitation down to ∼100 pM with just a 5 min-analysis.

Micropatterned aptasensors for continuous monitoring of cytokine release from human leukocytes

We report the development of a microdevice for detecting local interferon gamma (IFN-γ) release from primary human leukocytes in real time. Our microdevice makes use of miniature aptamer-modified electrodes integrated with microfluidics to monitor cellular production of IFN-γ. The aptamer species consists of a DNA hairpin molecule with thiol groups on the 3'-end for self-assembly onto Au electrodes. A redox reporter is covalently attached at the 5'-end for electrochemical sensing. This aptasensor has excellent sensitivity for IFN-γ (<60 pM detection limit) and responds to the target analyte in real time without additional washing or labeling steps. Aptamer-functionalized electrode arrays are fabricated on glass slides containing poly(ethylene glycol) (PEG) hydrogel patterns designed to expose glass regions adjacent to electrodes while protecting the remainder of the surface from nonspecific adsorption. The micropatterned substrates are integrated with PDMS microfluidic channels and incubated with T-cell-specific antibodies (Ab) (anti-CD4). Upon injection of blood, leukocytes are bound to Ab-modified glass regions in proximity to aptasensors. Cytokine release from captured cells is triggered by mitogenic activation and detected at the aptamer-modified electrodes using square wave voltammetry (SWV). The IFN-γ signal is monitored in real time with signal appearing as early as 15 min poststimulation from as few as 90 T cells. The observed IFN-γ release profiles are used to calculate an initial IFN-γ production rate of 0.0079 pg cell(-1) h(-1) upon activation. The work described here represents an important step toward development of aptasensors for immune cell analysis and blood-based diagnostics.

Photolabile micropatterned surfaces for cell capture and release

A method for capture and release of cells was developed using a photolabile linker and antibody-attached glass surface with a poly(ethylene glycol) (PEG)-pattern.

Electrochemical biosensors for on-chip detection of oxidative stress from immune cells

Seamless integration of biological components with electrochemical sensors is critical in the development of microdevices for cell analysis. The present paper describes the integration miniature Au electrodes next to immune cells (macrophages) in order to detect cell-secreted hydrogen peroxide (H(2)O(2)). Photopatterning of poly(ethylene glycol) (PEG) hydrogels was used to both immobilize horseradish peroxidase molecules onto electrodes and to define regions for cell attachment in the vicinity of sensing electrodes. Electrodes micropatterned in such a manner were enclosed inside poly(dimethylsiloxane) fluid conduits and incubated with macrophages. The cells attached onto the exposed glass regions in the vicinity of the electrodes and nowhere else on the non-fouling PEG hydrogel surface. A microfluidic device was converted into an electrochemical cell by placing flow-through Ag∕AgCl reference and Pt wire counter electrodes at the outlet and inlet, respectively. This microdevice with integrated H(2)O(2)-sensing electrodes had sensitivity of 27 μA∕cm(2) mM with a limit of detection of 2 μM. Importantly, this microdevice allowed controllable seeding of macrophages next to electrodes, activation of these cells and on-chip monitoring of H(2)O(2) release in real time. In the future, this biosensor platform may be utilized for monitoring of macrophage responses to pathogens or for the study of inflammatory signaling in micropatterned cell cultures.

Using personal glucose meters and functional DNA sensors to quantify a variety of analytical targets

Portable, low-cost and quantitative detection of a broad range of targets at home and in the field has the potential to revolutionize medical diagnostics and environmental monitoring. Despite many years of research, very few such devices are commercially available. Taking advantage of the wide availability and low cost of the pocket-sized personal glucose meter-used worldwide by diabetes sufferers-we demonstrate a method to use such meters to quantify non-glucose targets, ranging from a recreational drug (cocaine, 3.4 µM detection limit) to an important biological cofactor (adenosine, 18 µM detection limit), to a disease marker (interferon-gamma of tuberculosis, 2.6 nM detection limit) and a toxic metal ion (uranium, 9.1 nM detection limit). The method is based on the target-induced release of invertase from a functional-DNA-invertase conjugate. The released invertase converts sucrose into glucose, which is detectable using the meter. The approach should be easily applicable to the detection of many other targets through the use of suitable functional-DNA partners (aptamers, DNAzymes or aptazymes).

The use of glass substrates with bi-functional silanes for designing micropatterned cell-secreted cytokine immunoassays

It is often desirable to sequester cells in specific locations on the surface and to integrate sensing elements next to the cells. In the present study, surfaces were fabricated so as to position cytokine sensing domains inside non-fouling poly(ethylene glycol) (PEG) hydrogel microwells. Our aim was to increase sensitivity of micropatterned cytokine immunoassays through covalent attachment of biorecognition molecules. To achieve this, glass substrates were functionalized with a binary mixture of acrylate- and thiol-terminated methoxysilanes. During subsequent hydrogel photopatterning steps, acrylate moieties served to anchor hydrogel microwells to glass substrates. Importantly, glass attachment sites within the microwells contained thiol groups that could be activated with a hetero-bifunctional cross-linker for covalent immobilization of proteins. After incubation with fluorescently-labeled avidin, microwells fabricated on a mixed acryl/thiol silane layer emitted ∼ 6 times more fluorescence compared to microwells fabricated on an acryl silane alone. This result highlighted the advantages of covalent attachment of avidin inside the microwells. To create cytokine immunoassays, micropatterned surfaces were incubated with biotinylated IFN-γ or TNF-α antibodies (Abs). Micropatterned immunoassays prepared in this manner were sensitive down to 1 ng/ml or 60 pM IFN-γ. To further prove utility of this biointerface design, macrophages were seeded into 30 μm diameter microwells fabricated on either bi-functional (acryl/thiol) or mono-functional silane layers. Both types of microwells were coated with avidin and biotin-anti-TNF-α prior to cell seeding. Short mitogenic activation followed by immunostaining for TNF-α revealed that microwells created on bi-functional silane layer had 3 times higher signal due to macrophage-secreted TNF-α compared to microwells fabricated on mono-functional silane. The rational design of cytokine-sensing surfaces described here, will be leveraged in the future for rapid detection of multiple cytokines secreted by individual immune cells.

A zinc finger protein array for the visual detection of specific DNA sequences for diagnostic applications

The visual detection of specific double-stranded DNA sequences possesses great potential for the development of diagnostics. Zinc finger domains provide a powerful scaffold for creating custom DNA-binding proteins that recognize specific DNA sequences. We previously demonstrated sequence-enabled reassembly of TEM-1 β-lactamase (SEER-LAC), a system consisting of two inactive fragments of β-lactamase each linked to engineered zinc finger proteins (ZFPs). Here the SEER-LAC system was applied to develop ZFP arrays that function as simple devices to identify bacterial double-stranded DNA sequences. The ZFP arrays provided a quantitative assay with a detection limit of 50 fmol of target DNA. The method could distinguish target DNA from non-target DNA within 5 min. The ZFP arrays provided sufficient sensitivity and high specificity to recognize specific DNA sequences. These results suggest that ZFP arrays have the potential to be developed into a simple and rapid point-of-care (POC) diagnostic for the multiplexed detection of pathogens.

Stereomask lithography (SML): a universal multi-object micro-patterning technique for biological applications

The advent of biological micro-patterning techniques has given new impetus to many areas of biological research, including quantitative biochemical analysis, tissue engineering, biosensing, and regenerative medicine. Derived from photolithography or soft lithography, current bio-patterning approaches have yet to completely address the needs of out-of-cleanroom, universal applicability, high feature resolution, as well as multi-object placement, though many have shown great promise to precisely pattern one specific biomaterial. In this paper, we present a novel versatile biological lithography technique to achieve integrated multi-object patterning with high feature resolution and high adaptability to various biomaterials, referred to as stereomask lithography (SML). Successive patterning of multiple objects is enabled by using unique three-dimensional masks (i.e., the stereomasks), which lay out current micropatterns while protecting pre-existing biological features on the substrate. Furthermore, high-precision reversible alignment among multiple bio-objects is achieved by adopting a peg-in-hole design between the substrate and stereomasks. We demonstrate that the SML technique is capable of constructing a complex biological microenvironment with various bio-functional components at the single-cell resolution, which to the best of our knowledge has not been realized before.

Aptamer-based electrochemical biosensor for interferon gamma detection

In this paper, we describe the development of an electrochemical DNA aptamer-based biosensor for detection of interferon (IFN)-γ. A DNA hairpin containing IFN-γ-binding aptamer was thiolated, conjugated with methylene blue (MB) redox tag, and immobilized on a gold electrode by self-assembly. Binding of IFN-γ caused the aptamer hairpin to unfold, pushing MB redox molecules away from the electrode and decreasing electron-transfer efficiency. The change in redox current was quantified using square wave voltammetry (SWV) and was found to be highly sensitive to IFN-γ concentration. The limit of detection for optimized biosensor was 0.06 nM with linear response extending to 10 nM. This aptasensor was specific to IFN-γ in the presence of overabundant serum proteins. Importantly, the same aptasensor could be regenerated by disrupting aptamer-IFN-γ complex in urea buffer and reused multiple times. Unlike standard sandwich immunoassays, the aptasensor described here allowed one to detect IFN-γ binding directly without the need for multiple washing steps and reagents. An electrochemical biosensor for simple and sensitive detection of IFN-γ demonstrated in this paper will have future applications in immunology, cancer research, and infectious disease monitoring.

Micropatterning of Aptamer Beacons to Create Cytokine-Sensing Surfaces

Aptamer beacons are DNA or RNA probes that bind proteins or small molecules of interest and emit signal directly upon interaction with the target analyte. This paper describes micropatterning of aptamer beacons for detection of IFN-γ—an important inflammatory cytokine. The beacon consisted of a fluorophore-labeled aptamer strand hybridized with a shorter, quencher-carrying complementary strand. Cytokine molecules were expected to displace quenching strands of the beacon, disrupting FRET effect and resulting in fluorescence signal. The glass substrate was first micropatterned with poly(ethylene glycol) (PEG) hydrogel microwells (35 μm diameter individual wells) so as to define sites for attachment of beacon molecules. PEG microwell arrays were then incubated with avidin followed by biotin-aptamer-fluorophore constructs. Subsequent incubation with quencher-carrying complementary strands resulted in formation of DNA duplex and caused quenching of fluorescence due to FRET effect. When exposed to IFN-γ, microwells changed fluorescence from low (quencher hybridized with fluorophore-carrying strand) to high (quenching strand displaced by cytokine molecules). The fluorescence signal was confined to microwells, was changing in real-time and was dependent on the concentration of IFN-γ. In the future, we plan to co-localize aptamer beacons and cells on micropatterned surfaces in order to monitor in real-time cytokine secretion from immune cells in microwells.

On chip real time monitoring of B-cells hybridoma secretion of immunoglobulin

The secretions of molecules by cells are of tremendous interest for both fundamental insights studies and medical purposes. In this study, we propose a new biochip-based approach for the instantaneous monitoring of protein secretions, using antibody production by B lymphocytes cultured in vitro. This was possible thanks to the Surface Plasmon Resonance imaging (SPRi) of a protein biochip where antigen proteins (Hen Egg Lysozyme, HEL) were micro-arrayed along with series of control proteins. B cell hybridomas were cultured on the chip and the secretion of immunoglobulins (antibody) specific to HEL was monitored in real-time and detected within only few minutes rather than after a 30-60 min incubation with standard ELISA experiments. This fast and sensitive detection was possible thanks to the sedimentation of the cells on the biochip sensitive surface, where local antibody concentrations are much higher before dilution in the bulk medium. An other interesting feature of this approach for the secretion monitoring was the independence of the SPR response--after normalization--regarding to the density of the surface-immobilized probes. Such biosensor might thus pave the way to new tools capable of both qualitative and semi-quantitative analysis of proteins secreted by other immune cells.

An integrated micro-electro-fluidic and protein arraying system for parallel analysis of cell responses to controlled microenvironments

Living cells have evolved sophisticated signaling networks allowing them to respond to a wide array of external stimuli. Microfluidic devices, facilitating the analysis of signaling networks through precise definition of the cellular microenvironment often lack the capacity of delivering multiple combinations of different signaling cues, thus limiting the throughput of the analysis. To address this limitation, we developed a microfabricated platform combining microfluidic definition of the cell medium composition with dielectrophoretic definition of cell positions and protein microarray-based presentation of diverse signaling inputs. Ligands combined at different concentrations were spotted along with an extracellular matrix protein onto a glass substratum in alignment with an electrode array. This substratum was combined with a polydimethylsiloxane chip for microfluidic control of the soluble medium components, in alignment with the electrode and protein arrays. Endothelial cells were captured by dielectrophoretic force, allowed to attach and spread on the protein spots; and the signaling output of the NF-kappaB pathway in response to diverse combinations of IGF1 and TNF was investigated, in the absence and presence of variable dose of the pathway inhibitor. The results suggested that cells can be potently activated by immobilized TNF with IGF1 having a modulating effect, and the response could be abolished to different degrees by the inhibitor. This study demonstrates considerable potential of combining precise cell patterning and liquid medium control with protein microarray technology for complex cell signaling studies in a high-throughput manner.

Detecting interferon-gamma release from human CD4 T-cells using surface plasmon resonance

Cytokine secretion by leukocytes is an important indicator of immune response to pathogens and therefore has significant implications in disease diagnostics. Given heterogeneity of leukocyte subsets and the ability of multiple cell subsets to secrete the same cytokines, connecting cytokine production to a specific leukocyte subset is a distinct challenge. In the present paper we describe a strategy combining antibody (Ab)-based affinity cell separation and surface plasmon resonance (SPR) for capturing human CD4 T-cells and for label-free detection of cell-secreted interferon (IFN)-gamma--an important inflammatory cytokine. Human blood was introduced into a flow chamber modified with anti-CD4 Abs resulting in capture of CD4(+) T-cells. After mitogenic activation of cells inside the flow chamber, culture medium was routed onto an SPR chip modified with monoclonal IFN-gamma Abs. SPR signal observed in this experiment correlated with cytokine production by T-cells. The strategy of combining SPR detection with cell purification may be used in the future for label-free, sensitive detection of multiple cytokines or proteins secreted by the desired cell subset.

Development of an aptamer beacon for detection of interferon-gamma

Traditional antibody-based affinity sensing strategies employ multiple reagents and washing steps and are unsuitable for real-time detection of analyte binding. Aptamers, on the other hand, may be designed to monitor binding events directly, in real-time, without the need for secondary labels. The goal of the present study was to design an aptamer beacon for fluorescence resonance energy transfer (FRET)-based detection of interferon-gamma (IFN-gamma)--an important inflammatory cytokine. Variants of DNA aptamer modified with biotin moieties and spacers were immobilized on avidin-coated surfaces and characterized by surface plasmon resonance (SPR). The SPR studies showed that immobilization of aptamer via the 3' end resulted in the best binding IFN-gamma (K(d) = 3.44 nM). This optimal aptamer variant was then used to construct a beacon by hybridizing fluorophore-labeled aptamer with an antisense oligonucleotide strand carrying a quencher. SPR studies revealed that IFN-gamma binding with an aptamer beacon occurred within 15 min of analyte introduction--suggesting dynamic replacement of the quencher-complementary strand by IFN-gamma molecules. To further highlight biosensing applications, aptamer beacon molecules were immobilized inside microfluidic channels and challenged with varying concentration of analyte. Fluorescence microscopy revealed low fluorescence in the absence of analyte and high fluorescence after introduction of IFN-gamma. Importantly, unlike traditional antibody-based immunoassays, the signal was observed directly upon binding of analyte without the need for multiple washing steps. The surface immobilized aptamer beacon had a linear range from 5 to 100 nM and a lower limit of detection of 5 nM IFN-gamma. In conclusion, we designed a FRET-based aptamer beacon for monitoring of an inflammatory cytokine-IFN-gamma. In the future, this biosensing strategy will be employed to monitor dynamics of cytokine production by the immune cells.

Silicon photonic microring resonators for quantitative cytokine detection and T-cell secretion analysis

The ability to perform multiple simultaneous protein biomarker measurements in complex media with picomolar sensitivity presents a large challenge to disease diagnostics and fundamental biological studies. Silicon photonic microring resonators represent a promising platform for real-time detection of biomolecules on account of their spectral sensitivity toward surface binding events between a target and antibody-modified microrings. For all refractive index-based sensing schemes, the mass of bound analytes, in combination with other factors such as antibody affinity and surface density, contributes to the observed signal and measurement sensitivity. Therefore, proteins that are simultaneously low in abundance and have a lower molecular weight are often challenging to detect. By employing a more massive secondary antibody to amplify the signal arising from the initial binding event, it is possible to improve both the sensitivity and the specificity of protein assays, allowing for quantitative sensing in complex sample matrices. Herein, a sandwich assay is used to detect the 15.5 kDa human cytokine interleukin-2 (IL-2) at concentrations down to 100 pg/mL (6.5 pM) and to quantitate unknown solution concentrations over a dynamic range spanning 2.5 orders of magnitude. This same sandwich assay is then used to monitor the temporal secretion profile of IL-2 from Jurkat T lymphocytes in serum-containing cell culture media in the presence of the entire Jurkat secretome. The same temporal secretion analysis is performed in parallel using a commercial ELISA, revealing similar IL-2 concentration profiles but superior precision for the microring resonator sensing platform. Furthermore, we demonstrate the generality of the sandwich assay methodology on the microring resonator platform for the analysis of any biomolecular target for which two high-affinity antibodies exist by detecting the approximately 8 kDa cytokine interleukin-8 (IL-8) with a limit of detection and dynamic range similar to that of IL-2. This work demonstrates the first application of silicon photonic microring resonators for detecting cellular secretion of cytokines and represents an important advance for the detection of protein biomarkers on an emerging analytical platform.

Lensfree holographic imaging of antibody microarrays for high-throughput detection of leukocyte numbers and function

Characterization of leukocytes is an integral part of blood analysis and blood-based diagnostics. In the present paper, we combine lensless holographic imaging with antibody microarrays for rapid and multiparametric analysis of leukocytes from human blood. Monoclonal antibodies (Abs) specific for leukocyte surface antigens (CD4 and CD8) and cytokines (TNF-alpha, IFN-gamma, IL-2) were printed in an array so as to juxtapose cell capture and cytokine detection antibody (Ab) spots. Integration of Ab microarrays into a microfluidic flow chamber (4 muL volume) followed by incubation with human blood resulted in capture of CD4 and CD8 T-cells on specific Ab spots. On-chip mitogenic activation of these cells induced release of cytokine molecules that were subsequently captured on neighboring anticytokine Ab spots. The binding of IL-2, TNF-alpha, and IFN-gamma molecules on their respective Ab spots was detected using horseradish peroxidase (HRP)-labeled anticytokine Abs and a visible color reagent. Lensfree holographic imaging was then used to rapidly ( approximately 4 s) enumerate CD4 and CD8 T-lymphocytes captured on Ab spots and to quantify the cytokine signal emanating from IL-2, TNF-alpha, and IFN-gamma spots on the same chip. To demonstrate the utility of our approach for infectious disease monitoring, blood samples of healthy volunteers and human immunodeficiency virus (HIV)-infected patients were analyzed to determine the CD4/CD8 ratio, an important HIV/AIDS diagnostic marker. The ratio obtained by lensfree on-chip imaging of CD4 and CD8 T-cells captured on Ab spots was in close agreement with conventional microscopy-based cell counting. The present paper, describing tandem use of Ab microarrays and lensfree holographic imaging, paves the way for future development of miniature cytometry devices for multiparametric blood analysis at the point of care or in a resource-limited setting.

Detecting cytokine release from single T-cells

The cytokine production by leukocytes correlates with body's ability to mount an immune response and therefore has high diagnostic value. In the present study we employed microfabricated surfaces to capture T-cells from minimally processed human blood, arrange these cells into a single cell array, and then detect interferon (IFN)-gamma released from individual cells. The fabrication of cell capture surfaces started with coating a silane-modified glass slide with a uniform layer of poly(ethylene glycol) (PEG) hydrogel. The hydrogel-coated slide was lyophilized and then incubated with a mixture of monoclonal anti-IFN-gamma and anti-CD4 antibodies (Abs). To define sites for single cell attachment, PEG hydrogel microwells (20 microm diameter) were photolithographically patterned on top of the Ab-containing hydrogel layer. This micropatterning process resulted in fabrication of PEG hydrogel microwells with Ab-decorated bottom and nonfouling walls. To minimize the blood volume requirement and to precisely define shear stress conditions, the engineered surface was enclosed inside a PDMS-based microfluidic device. Introduction of red blood cell (RBC) depleted whole human blood followed by controlled washing led to the isolation of individual CD4 T-cells within PEG microwells. Mitogenic activation and immunofluorescent staining performed inside the microfluidic chamber revealed IFN-gamma cytokine signal colocalized with specific T-cells. The device and process presented here will be expanded in the future to enable multiparametric functional analysis of immune cells organized into high density single cell arrays.

Review: Microfluidic applications in metabolomics and metabolic profiling

Metabolomics is an emerging area of research focused on measuring small molecules in biological samples. There are a number of different types of metabolomics, ranging from global profiling of all metabolites in a single sample to measurement of a selected group of analytes. Microfluidics and related technologies have been used in this research area with good success. The aim of this review article is to summarize the use of microfluidics in metabolomics. Direct application of microfluidics to the determination of small molecules is covered first. Next, important sample preparation methods developed for microfluidics and applicable to metabolomics are covered. Finally, a summary of metabolomic work as it relates to analysis of cellular events using microfluidics is covered.

Micropatterned co-cultures of T-lymphocytes and epithelial cells as a model of mucosal immune system

Gut-associated lymphoid tissue is a major target and reservoir of human immunodeficiency virus (HIV)-infected T-cells. Our studies seek to recapitulate, in vitro, interactions between HIV-infected T-lymphocytes and intestinal epithelial cells in order to investigate the mechanisms underlying the disruption of normal epithelial cell and barrier function. Here, we describe a novel approach for creating co-cultures of healthy or HIV-infected T-lymphocytes (Jurkat) and human intestinal epithelial (HT-29) cells where both cell types are positioned on the same surface in a price spatial configuration (micropattern). This co-culture method simplified observation/monitoring of the two cell types and was particularly suited for laser microdissection-based retrieval of the desired cells for downstream gene expressions studies. DNA microarray analysis of epithelial cells retrieved from co-cultures with HIV-1-infected vs. uninfected Jurkat cells revealed that epithelial cells from HIV-infected co-cultures exhibited gene expression patterns consistent with disruption of epithelial barrier formation. Overall, the micropatterned co-culture system described here is envisioned as a valuable new tool for delineating how HIV and other infections contribute to dysfunction of mucosal epithelium.